Canine and feline thyroid disease (Proceedings)

B. May be able to palpate a thyroid nodule before the T4 is elevated. Recheck T4 3 every 3 – 6 months or when signs occur.

C. Occasionally cats with hyperthyroidism may have a T4 in the normal range at the time of sampling. This is especially true in cats with mild hyperthyroidism. A second sample may be needed in those cats with strong clinical evidence of thyrotoxicosis. The second sample should be taken a few days to weeks later, as more pronounced fluctuations in thyroid hormone levels occur over days rather than hours. Nonthyroidal illness may also result in highnormal serum T4 concentrations even in the face of hyperthyroidism. Following correction of the underlying illness or discontinuation of medications, T4 levels will increase into the hyperthyroid range.

In animals in which hyperthyroidism is suspected, but the basal T4 levels are consistently normal, four additional tests can be considered.

1. T3 Suppression Test:

A. Basis of the Test: The normal pituitarythyroid axis will be suppressed following supplementation with T3. A decrease in TSH concentration will lead to a decrease in T4 levels.

B.
Performing the Test

1. Determine basal T4 level.

2. Administer T3 (25 ug) every 8 hours for two days, giving the last dose on the morning of day 3.

3. Determine T3 and T4 concentrations 4 hours following the last dose of T3.

4. Normal cats:

a) T4 levels suppress greater than 50% from pretreatment value.

2. TRH Stimulation Test:

A. Basis of the Test: TRH is the hypothalamic peptide that regulates TSH release from the pituitary. TSH response to TRH is blunted in patients with hyperthyroidism.

B. Performing the Test:

1. Obtain basal T4 level.

2. Administer 0.1 mg/kg TRH IV.

3.
Obtain 4 hour post TRH T4.

4. Normal cats:

a)
Twofold rise in T4 post TRH

b)
Hyperthyroid cats have minimal to no increase in T4.

3. Free T4

A. In cats where the TT4 is in the upper 50% of the basal resting range, an elevated fT4ED in the face of clinical signs is highly predictive of hyperthyroidism. Use of fT4ED should not be used as the initial screening test as some euthyroid senior cats have have elevated fT4ED. Due to the simplicity of the test, fT4ED should be the first line test in diagnosing cats with hormonally occult (normal TT4) hyperthyroidism.

4.
Imaging

A. Technetium scans may be helpful in hormonally borderline cases where bilateral uptake is clearly increased or unilateral disease is present.

Summary

In the last few years it is still apparent that the best test to use in the initial approach to the patient with hyperthyroidism is measurement of total T4 (TT4) concentrations. TT4 testing is simple and inexpensive and will provide the correct diagnosis in the majority of feline patients presented for evaluation. However, we are now faced with attempting to diagnose or confirm hyperthyroidism in cats that are asymptomatic, have only mild clinical signs, and/or have concurrent illness that may affect accurate laboratory assessment of thyroid function. These cases can be very challenging though recent work seems to indicate that measurement of free T4 by equilibrium dialysis (fT4ED) represents the logical next step (though it should be emphasized, not the first step) in the approach to these patients. This approach will likely eliminate the need for additional expensive or problematic tests such as TRH stimulation and T3 suppression testing. We also have seen recent work on the effects of thyrotoxicosis on bone and calcium metabolism and how hyperthyroidism can affect our laboratory assessment of concurrent diseases such as diabetes.